The present invention generally relates to optical scanning apparatus, and more particularly to an optical scanning apparatus which is used in an image forming system such as a laser printer, a facsimile machine, a copying machine or the like for forming or reproducing an image through an electrophotographic technique.
There are several prior art image forming techniques which have been developed to achieve an image density modification. A conventional apparatus as disclosed in Japanese Published Patent Application No. 61-66467 is directed to changing a diameter of a laser beam by controlling a power to drive a semiconductor laser. However, the change in the beam spot diameter obtained from an ordinary laser beam power is very limited, and usually it is difficult to make a beam diameter twice as large as the original diameter without causing a problem about image quality. Also, for example, it is difficult to modify the density of the image to be formed from 600 dpi (dots per inch) to 300 dpi while keeping a clear image with this prior art apparatus. And an extremely large change in the drive current of the semiconductor laser device is required for increasing and decreasing the laser beam power, which may result in the deterioration of the semiconductor laser device due to a great amount of electric current being consumed.
Another conventional apparatus as disclosed in Japanese Published Patent Application No. 61-113018 is directed to the provision of a variable aperture or beam limiting device in an optical path and the provision of a control device for controlling a size of an opening of the variable aperture to vary a size of a beam spot produced on a photosensitive medium. However, there is a problem in that a reduction in the quantity of incoming light, due to the use of such a variable aperture, leads to an undesirable variation of the quantity of light being exposed on the photosensitive medium.
Still another prior art apparatus as disclosed in Japanese Published Patent Application No. 61-33073 is directed to varying a diameter of a scanning beam spot by replacement of a plurality of beam diameter modification lenses positioned in an optical path. In this prior art apparatus, replacing such beam diameter modification lenses is a burdensome task, causing inconvenience for an operator who operates the apparatus.
Japanese Published Patent Application No. 56-71374 discloses an image recording method which is carried out with an image recording apparatus which is capable of generating several levels of optical density with pulse duration modulated signals obtained from electric input signals indicating a subject image including halftones, so that the information of the subject image containing multilevel gradation is reproduced. In this prior art recording apparatus, the beam diameter may be changed effectively only in a horizontal scanning direction. However, the drawback of this apparatus is that the beam diameter in a vertical scanning direction cannot be changed and so a multilevel gradation recording is not achieved in a manner adequate for commercial practice. The horizontal scanning direction described herein refers to a direction of scanning by a scanning mechanism from left to right or vice versa of a subject image as when reading a page of print, while the vertical scanning direction described herein refers to a direction of scanning due to a relative movement by a target surface from top to bottom or vice versa of a subject image with regard to an optical path along which a beam of light passes.